A glass-ceramic is a material having at least one crystalline phase thermally developed in a uniform pattern throughout at least a portion of a glass precursor. Glass-ceramics have been known for over 30 years since being described in U.S. Pat. No. 2,920,971 (Stookey). They find application in diverse areas, an area of particular interest being the fabrication of articles used in the preparation and serving of food. Such articles include cookware, bakeware, tableware and flat cooktops.
In general, production of a glass-ceramic material involves three major steps:
1. Melting a mixture of raw materials, usually containing a nucleating agent, to produce a glass. PA1 2. Forming an article from the glass and cooling the glass below its transformation range. PA1 3. Crystallizing ("ceramming") the glass article by an appropriate thermal treatment.
The thermal treatment usually involves a nucleating step at a temperature slightly above the transformation range. This is followed by heating to a somewhat higher temperature to cause crystal growth on the nuclei.
Crystallization of glasses in the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 composition field generally provides highly crystallized glass-ceramics. The primary crystal phase depends on glass composition and heat treatment. It may be a transparent beta-quartz solid solution, or an opaque beta-spodumene solid solution.
Beta-quartz and beta-spodumene solid solution glass-ceramics customarily contain TiO.sub.2 as a nucleating agent. Optionally, the TiO.sub.2 may be partially, or wholly, substituted by ZrO.sub.2. The appearance of such glass-ceramics can be varied by varying composition and/or heat treatment. Thus, transparent, translucent, or opaque glass-ceramics (which may be water-white, translucent, opaque white, or variously colored) are all possibilities as described in the prior art.
The widest use of Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 glass-ceramic materials has been in the field of culinary ware. For over three decades, Corning Glass Works, now Corning Incorporated, has marketed opaque white cooking utensils under the trademark CORNING WARE. The Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 glass-ceramic of this ware has a beta-spodumene crystal phase. In general, this opaque glass-ceramic is crystallized at high temperatures, which may reach 1150.degree. C., to develop large, beta-spodumene solid solution crystals that render the article opaque.
U.S. Pat. No. 3,157,522 (Stookey) discloses a family of glasses that may be crystallized to produce glass-ceramics having an opaque, beta-spodumene crystal phase. This family consists essentially, in percent by weight calculated on an oxide basis, of 55-75% SiO.sub.2, 3-6% TiO.sub.2, 2-6.5% Li.sub.2 O and 12-36% Al.sub.2 O.sub.3, the weight ratio LiO.sub.2 :Al.sub.2 O.sub.3 being 0.1 to 0.6 and the essential ingredients totaling at least 95%. It has subsequently been learned that ZrO.sub.2 may replace TiO.sub.2, at least in part, as a nucleating agent.
U.S. Pat. No. 5,070,045 (Comte et al.) discloses transparent, glass-ceramic plates wherein the predominant crystal phase in the glass-ceramics is beta-quartz solid solution. These plates use 0.1-1.0% of a colorant selected from CoO, NiO, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, MnO.sub.2, and V.sub.2 O.sub.5. The patent is primarily concerned with V.sub.2 O.sub.5 which contributes to minimal distortion while giving a black aspect in reflection and a reddish brown tint in transmission. The Comte et al. compositions consist essentially, in weight percent, as calculated on the oxide basis, of:
______________________________________ SiO.sub.2 65-70 MgO + BaO + SrO 1.1-2.3 Al.sub.2 O.sub.3 18-19.8 ZrO.sub.2 1.0-2.5 Li.sub.2 O 2.5-3.8 As.sub.2 O.sub.3 0-1.5 MgO 0.55-1.5 Sb.sub.2 O.sub.3 0-1.5 ZnO 1.2-2.8 As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0.5-1.5 TiO.sub.2 1.8-3.2 Na.sub.2 O 0-&lt;1.0 BaO 0-1.4 K.sub.2 O 0-&lt;1.0 SrO 0-1.4 Na.sub.2 O + K.sub.2 O 0-&lt;1.0 BaO + SrO 0.4-1.4 ##STR1## &gt;1.8 ______________________________________
The Comte et al. base glasses have been found effective for use in producing both opaque and transparent, as well as colored and uncolored, glass-ceramic products from a single glass melting tank. In accomplishing this, a technique known as a forehearth coloring system, or colorcell, is employed.
Use of that technique makes it possible to obtain different colors, as well as the opaque product, with a single base glass composition for the precursor glass. That facilitates changing from one product to another with a single melting unit. It is also possible to melt one precursor base glass, and then make subsequent additions in the furnace forehearth to achieve different colors.
U.S. Pat. No. 4,977,110 (Amundson, Jr. et al.) discloses a family of glass-ceramic compositions within the broad scope of the Stookey patent. Those glass-ceramics are opaque, beige-tinted and have a CeO.sub.2 content of 1.5-2.75% by weight. They provided acceptable colors, but a desire has arisen to soften or lighten the color. Also, it would be desirable to limit the use of CeO.sub.2 which is relatively expensive and which, in such large amounts, gives rise to compatibility problems in color cell practice.
The present invention provides a color package that achieves both desired ends. In particular, it provides a soft beige color in beta-spodumene solid solution glass-ceramic ware.